Patients suffering from CHF, and other conditions that cause pulmonary edema, are typically treated with diuretic drugs to reduce the fluid in their lungs. But these drugs can also cause kidney damage and other medical problems, so it is generally advantageous, in treating these patients, to monitor the level of pulmonary edema, in order to maintain an optimized treatment regime. Levels of pulmonary edema may be designated by a scale that ranges from the least severe cases to the most severe. Bio-impedance techniques offer a non-invasive and inexpensive method for monitoring fluid volume in the lungs.
Systems for bio-impedance monitoring of pulmonary edema, developed in the 1970s, are described by R. N. McCarthy and D. Dosh, “Assessment of pulmonary edema in acute congestive heart failure with impedance cardiography,” J. Am. Osteo. Assoc. 74, 879 (1975), and by M. R. Khan et al, “Quantitative electrical-impedance plethysomography for pulmonary oedema,” Med. Bio. Eng. Comp. 15, 627-633 (1977). These systems used two or four electrodes placed on the outside of the body at the upper and lower ends of the thorax. A commercially available system similar to these, the BioZ system sold by Cardiodynamics, is described by B. H. Greenberg et al, “Reproducibility of impedance cardiography hemodynamic measures in clinically stable heart failure patients,” CHF 6, 19-26 (2000). The systems described in the above referenced documents generally yield only an estimate of the average overall thorax impedance, which can be influenced by factors, for example the thickness of surrounding fat layers, other than pulmonary edema.
Another kind of bio-impedance system for lung monitoring is usable only for patients who have implanted pacemakers or defibrillators having electrical leads permanently placed inside the heart, typically in the right atrium and the right ventricle. One such system, the Insync Sentry, made by Medtronic, is described by L. Wang et al, “Prediction of CHF hospitalization by ambulatory intrathoracic impedance measurement in CHF patients is feasible using pacemaker or ICD lead systems,” PACE 26, 959 (2003). In this system, current is injected between the right ventricle lead and the case of the pacemaker or defibrillator, and the impedance between these two points is measured. This impedance is found to be well correlated with the overall amount of fluid in the lungs, and is insensitive to external factors such as the thickness of fat layers surrounding the thorax.
A. Belalcazar and R. P. Patterson, “Improved lung edema monitoring with coronary vein pacing leads: a simulation study,” Physiol. Meas. 25, 475-487 (2004), describes a bio-impedance system using a new type of implanted pacemaker or defibrillator. The new pacemaker or defibrillator has a lead in the left ventricle coronary vein for improved pacemaking or defibrillating functioning, in addition to leads in the right atrium and right ventricle. Like many pacemakers and defibrillators, the described pacemaker or defibrillator has its case implanted in the left pre-pectoral position. Measuring impedance between each of these leads and the case allows changes in impedance due to pulmonary edema to be evaluated independently of changes in impedance due to intravascular fluid. The left ventricle coronary vein lead is said by Belalcazar and Patterson to be important for monitoring edema, because impedance measurements using this lead are more sensitive to pulmonary edema, and less sensitive to the volume of intravascular fluid, than impedance measurements using the right atrium and right ventricle leads.
Impedance measurements using any of the leads in the devices described by Belalcazar and Patterson are said to be sensitive only to left lung edema, not to right lung edema, in patients where the degree of edema is very different in the two lungs (unilateral edema). Whereas it is pointed out that for most pulmonary edema patients, fluid is distributed approximately uniformly in both lungs, pulmonary edema concentrated asymmetrically in the right lung does occur and can be a symptom of mitral regurgitation. The authors state that, for detecting pulmonary edema, “The right lung can only be monitored with a right pectoral implant.”
Ziochiver, Rosenfeld and Abboud, in “Induced Current Electrical Impedance Tomography: A 2-D Theoretical Simulation,” IEEE Transactions on Medical Imaging 22, 1550-1560 (2003), describe an impedance imaging system in which eight or more electrodes are arranged around the outside of the chest. Impedances are measured between different pairs of the electrodes, and the measurements are used to calculate an impedance image of the chest, with sufficiently high resolution to “see” each of the two lungs and the heart. Abboud is also an inventor on published U.S. application 2004/0006279-A1, which describes a similar system, and on the present application.
The disclosures of all the patents and publications mentioned above are incorporated herein by reference.